1. Field of the Invention
The present invention relates to a powdering device for spraying an anti-setoff powder onto a printed surface of a printed sheet made by a sheet-fed printing press.
2. Description of the Related Art
In a sheet-fed printing press, such as a sheet-fed offset printing press, printed sheets, that is, printing products are transferred to and piled on a pile board. When printing products are piled with ink on the printing products being not sufficiently dried, a printing trouble called a blocking (offset) often occurs. In the blocking, a part of an image on the front side of a preceding printing product is transferred, due to the weight of a following printing product piled on the preceding printing product, to the back side of the following printing product. For the purpose of preventing the blocking trouble, a powder made primarily from cornstarch or the like is sprayed onto printing products being transferred to a pile board after completion of printing.
It is generally required that the amount of powder sprayed onto a printing product be very small and be uniform as time elapses. If the amount of sprayed powder is less than a required amount, the blocking cannot be sufficiently prevented from occurring. By contrast, if the amount of sprayed powder exceeds the required amount, a large amount of the powder is attached to a printing product to thus roughen the texture of the surface of the printing product, thereby deteriorating the quality of the printing product. Moreover, an excessive amount of the powder is scattered around, eventually polluting the environment. For this reason, it is necessary that an adequate but not excessive amount of powder to be sprayed should be stably and continuously fed.
Japanese Patent No. 3825298 discloses an example of a powder spraying device with the amount of powder to be sprayed taken into consideration. The spraying device described in Japanese Patent No. 3825298 has a structure in which an appropriate-amount distribution roll having multiple grooves formed in the peripheral surface thereof is rotated about a horizontal axis in a housing. A storage container is positioned above the housing, and a jetting device for sucking the powder with a negative pressure so as to jet the powder downwards is connected to the lower side of the housing. The powder in the storage container enters the grooves of the appropriate-amount distribution roll. Then, the powder having entered the grooves is moved along the housing in association with the rotation of the appropriate-amount distribution roll about the horizontal axis, then falling down from the grooves at a lower portion so as to be discharged. The powder falling down from below is carried out in an air flow generated by the jetting device, and is discharged downward. The powder thus discharged downward passes through unillustrated piping connected with unillustrated tubes, thereby being transported to unillustrated nozzles for jetting the powder onto a surface of a printing product.
In addition, Japanese Patent Application Publication No. Hei 2-76738 discloses another example of a powder spraying device. In this powder-spraying device, a powder storage body is disposed above predetermined-powder-amount extruding screws which rotate about the vertical axis, while a mixing chamber for mixing the powder with air and a nozzle for discharging the powder mixed with the air are disposed below the screws. The powder in the powder storage body is caused to fall down into the mixing chamber by the predetermined-powder-amount extruding screws. The powder is then mixed with air in the mixing chamber, and thereafter, is sprayed onto a printing product through the nozzle.
The powder spraying device disclosed in Japanese Patent No. 3825298, however, has the following problem. This powder spraying device is configured so that the rotation of the appropriate-amount distribution roll causes the powder to be fed into the annular grooves about the horizontal axis. However, the weight of the powder inside the storage container is always applied to the powder stuffed in the grooves, so that the density of the powder stuffed in the space in the grooves is increased. Since the amount of the powder in the storage container constantly varies, the weight applied to the powder stuffed in the grooves changes. Accordingly, the degree of increase in the density of the powder in the space in the grooves is changed in accordance with the change in weight applied to the powder stuffed in the grooves. Since the powder stuffed in the grooves is transferred in such state by the friction with the peripheral surface of the appropriate-amount distribution roll, an appropriate amount of the powder cannot be precisely distributed.
On the other hand, the powder spraying device according to Japanese Patent Application Publication No. Hei 2-76738 has the following problem. This powder spraying device is configured so that the powder stuffed between the predetermined-powder-amount extruding screws is supplied. However, the weight of the powder in the powder storage is always applied to the powder stuffed between the predetermined-powder-amount extruding screws, so that the density of the powder stuffed in the space between the predetermined-powder-amount extruding screws is increased. Since the amount of the powder in the powder storage constantly varies, the weight applied to the powder stuffed between the screws changes. Accordingly, the degree of increase in the density of the powder in the space between the screws is changed in accordance with the change in weight applied to the powder stuffed between the screws. Since the powder stuffed between the screws in such state is transferred by the friction with the peripheral surface of the predetermined-powder-amount extruding screws, a constant amount of the powder cannot be precisely distributed. Moreover, the powder may possibly fall down excessively due to its own weight. As a result, it is further difficult to precisely distribute the powder.
Note that, consider that a stirring plate for loosening the powder in the storage container is disposed in the powdering device described in the Japanese Patent No. 3825298. In this case, it is necessary, for example, to provide a mechanism for converting the rotation about the horizontal axis into the rotation about the vertical axis as described in Japanese Patent Application Publication No. Hei 2-76738.